Sammendrag
This report presents the results from “Årsovervåkingen med FerryBox”, the yearly monitoring undertaken for “Fagrådet for indre Oslofjord” in 2017 with the use of NIVA FerryBox system for continuous measurements and sampling. The nutrients conditions were not classified, but the values were discussed using the Norwegian classification of water according to the water framework directive. In the surface layer, the winter values for total phosphorus (μg P/L) were relatively high, consistent with moderate to poor conditions, while the condition was good for total nitrogen (μg N/L). During winter there were varying concentrations of nitrate (μg N/L) and phosphate (μg P/L), with average values consistent with a moderate condition. For total nitrogen (μg N/L) conditions were good both during summer and winter. There were low values of ammonium (μg N/L). In summer, the values were consistent with good or very good conditions for total nitrogen and phosphorus. The concentration of chlorophyll a was low in the first months of the year. The spring bloom started in the beginning March and had a relatively high biomass, measured at 6.5 μg/L in from a water sample at March 7. The chlorophyll a fluorescence peak (> 10 μg/L) was measured shortly after this water sample was taken, so the sample did not “catch” the peak of the bloom. There was a relatively steady growth of algae until mid- une, before the concentrations were relatively low again for the rest of the summer. There was a small bloom (~ 2-4 μg/L) again in October. The Chl a fluorescence showed high biomass closer to Drøbak at the end of August, and this was not seen in water samples from Dk1. The temperature in Inner Oslofjord varied as a result of seasonal heating during the summer and cooling in the winter months. The salinity was very high (sometimes> 30) for a longer period during the autumn in 2016, and this period lasted also into 2017 until about February to March. The salinity was influenced by heavy rainfall during the summer and autumn, as seen when compared to the influx of river water in Drammenselva and Sandvikselva. Lower salinity in the outer fjord has great influence also into the inner fjord. During the first part of March 2017 the diatoms peaked in biomass and Rhizosolenia spp. including R. hebetata f. semispina which is a common species during the spring bloom in Oslofjorden dominated.
The diatom biomass decreased throughout spring and early summer while the amount of dinoflagellates began to rise sharply from the end of March with a bloom of Prorocentrum cf. balticum. There were large populations of P. cf. balticum until the beginning of May and it was present already from mid-January. Tripos muelleri was numerous in late May and until the end of June. From July onwards the dinoflagellat biomass decreases and continued to do so during late summer and autumn until it increases again in late September. In October, a small peak dominated by Tripos muelleri was recorded. All the common Tripos-species occurred from the end of September and throughout the last months of the year. At the end of July there is a small biomass peak dominated by Proboscia alata, and one more at the end of September dominated by Chaetoceros curvisetus.
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